Field of the Invention
The present disclosure relates to a carrier for a developer of an electrostatic latent image, and a two-component developer, a replenishing developer, an image forming apparatus, and a toner stored unit all of which use the carrier.
Description of the Related Art
In the course of image formation in an electrophotographic system, an electrostatic latent image is formed on an electrostatic latent image bearer, such as a photoconductive material, a charged toner is deposited on the electrostatic latent image to form a toner image, and the toner image is then transferred to a recording medium and fixed to form an output image. Recently, a technology related to photocopiers or printers using an electrophotographic system has been rapidly sifted from monochrome image formation to full-color image formation. A market for full-color image formation is expected to expand even more.
A two-component developer used for an electrophotographic system is composed of a toner and a carrier. The carrier is a bearing material that is mixed and stirred with the toner separately supplied inside a developing chamber to impart desired charge to the toner, and to transport the charged toner to an electrostatic latent image on a photoconductor to thereby form a toner image.
As a carrier, a known carrier is a carrier having a resin including carbon black formed on a surface of a carrier as a coating layer for the purpose of preventing filming of a toner, forming a uniform surface, preventing oxidization of the surface, preventing reduction in moisture sensitivity, extending a service life of a developer, preventing deposition on a surface of a photoconductor, protecting a photoconductor from scratches or abrasion, controlling a polarity of charging, and adjusting a charge amount.
Although excellent images can be formed with the above-mentioned carrier, there is a problem that an image quality decreases due to peeling of a coating layer of the carrier, as the number of sheets copied increases. Moreover, there is a problem that color staining occurs because the coating layer is peeled off from the carrier, or carbon black is fallen off from the coating layer. Generally, titanium oxide and zinc oxide are known as alternative materials of carbon black, but such materials demonstrate an insufficient effect of lowering volume resistivity.
Japanese Unexamined Patent Application Publication No. 11-202560 discloses a carrier to which a coating layer containing antimony-doped tin oxide (ATO) as a needle-shaped conductive powder is formed. Japanese Unexamined Patent Application Publication No. 2006-39357 discloses a carrier, in which a coating layer containing conductive particles is formed on a surface of each base particle, where each conductive particle includes a laminate of a tin dioxide layer and an indium oxide layer containing tin dioxide.
Moreover, it has been know that a coating layer including two different types of particles is disposed on a core. For example, Japanese Unexamined Patent Application Publication No. 11-184167 discloses a coated carrier in which a first coating layer disposed just above a core includes a needle-like or scaly conductive powder and a second coating layer disposed above the first coating layer includes a particulate conductive powder. Japanese Unexamined Patent Application Publication No. 07-286078 discloses a carrier, in which a coating layer disposed on a carrier core includes a binder resin, first particles an average particle of which is equal to or larger than a film thickness of the coating layer, and second particles an average particle of which is equal to or smaller than the film thickness of the coating layer, and discloses, as an example that volume resistivity of the second particles is 1.0×1012Ω·cm or less (Claim 1 of Japanese Unexamined Patent Application Publication No. 07-286078), an example (Example 1) of a carrier, in which a coating layer is formed on each ferrite core with a coating liquid including 1,500 parts by mass of alumina particles (volume average particle diameter: 0.35 μm, volume resistivity: 1.0×1014Ω·cm), 6,000 parts by mass of titanium oxide (volume average particle diameter: 0.015 μm, volume resistivity: 1.0×106Ω·cm), and 1,950 parts by mass of curable acrylic resin (solid content: 50%). Japanese Unexamined Patent Application Publication No. 2006-39357 discloses a technique for adjusting resistance using conductive fillers composed of tin dioxide and indium oxide.
Moreover, Japanese Unexamined Patent Application Publication No. 2010-117519 also discloses a carrier having a coating layer including first conductive particles, in which carbon is disposed on surfaces of tin oxide particles, and second conductive particles, in which surfaces of metal oxide particles and/or metal salt particles are conduction-treated.
Japanese Unexamined Patent Application Publication No. 2012-048167 discloses a carrier having a coating layer including magnetite particles.
Japanese Unexamined Patent Application Publication No. 2012-053421 discloses a carrier having a coating layer including an oxygen-deficient tin oxide-coated barium sulfate powder (product name: Pastran 4310, available from MITSUI MINING & SMELTING CO., LTD.).